1. Field of the Invention
The present invention relates to an optical integrated circuit and, more particularly, to an optical integrated circuit used for an optical fiber communication system and for an optical disk device.
2. Description of the Related Art
A conventional interference circuit such as a wavelength converter has a pair of gain regions which are formed parallel to each other on a semiconductor substrate. The gain region has an optical amplifier which amplifies an incident light by applying forward current between the electrodes, and performs an operation using nonlinear optical effect. At least two Y optical couplers are formed on the semiconductor substrate. The optical coupler and the gain region are connected by an optical waveguide.
An incident light into a semiconductor optical amplifier circuit is demultiplexed by an optical coupler, transmitted through an optical waveguide to each gain region, and then amplified in the each gain region. The light amplified in the gain region is transmitted to another optical coupler and multiplexed, and then emitted from the circuit as an emitting light (IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 13, No. 6, JUNE, 2001, pp.600–602)
When the length of the gain region of the circuit is relatively long and therefore the gain is large, an unintended laser oscillation (i.e., parasitic oscillation) is occurred by the reflection of spontaneous emission light generated in the gain region on the optical couplers provided on the opposite sides of the gain region. Therefore, a problem may occur that the laser oscillation introduces a noise into the laser signal. Where the optical coupler includes an embedded optical waveguide (e.g., 1.5 μm in width, 200 nm in thickness) having an InGaAsP core layer formed on an InP substrate, the residual reflectance is about one percent for the light of 1.5 μm in wave length, for example.
The gain in the gain region of the circuit should be made smaller by shortening the length of the gain region, for example, to eliminate the parasitic oscillation, which results in that a higher amplification can not be obtained in the circuit.